1.Field of the Invention
This invention relates to an improved cryogenic liquefied gas container, or a cold evaporator (hereinafter referred to as CE), to be installed on a site for storing liquefied argon gas, liquefied oxygen gas, liquefied nitrogen gas or liquefied carbon dioxide gas for supplying chemical plants, semiconductor manufacturing plants, argon welding facilities or the like with Ar, O.sub.2, N.sub.2, CO.sub.2 or the like, whichever appropriate or for storing, for example, liquefied oxygen gas and liquefied nitrogen gas, which are fed to an artificial air producing apparatus that mixes them to produce artificial air and feed it to hospitals and other medical facilities for artificial respirators or the like.
2. Prior Art
FIG. 4 of the accompanying drawings schematically illustrates a known CE. Referring to FIG. 4, an outer shell B typically made of rolled steel contains therein an inner tank C typically made of a material that shows satisfactory strength and tenacity and does not become brittle at cryogenic temperature with a vacuum insulation layer D made of a powdery insulating material (e.g., perlite) having a low thermal conductivity interposed therebetween, an inner piping system E linked to the inner tank C being extended to the outside through the vacuum insulating layer D and the outer shell B and coupled to an outer piping system F fitted to the external shell B.
Thus, the inner tank C can store only a type of cryogenic liquefied gas, e.g. liquefied oxygen gas, and therefore, for supplying artificial air, at least a storage tank for storing liquefied oxygen gas and another storage tank for storing liquefied nitrogen gas have to be installed on a given site A. Installing two tanks on a single site is a very space-taking operation and may not be feasible from the viewpoint of availability of land particularly for hospitals located in downtown areas.
With a known CE as described above and illustrated in FIG. 4, the liquefied gas LG brought in by a tank truck is fed into the inner tank C from a feed-in line L.sub.1 by way of a liquid inlet valve a of the outer piping system F and lower inner tank pipes E.sub.1 and E.sub.1 ' of the inner piping system E so that the liquefied gas LG may be introduced into the inner tank C both from the bottom and from the top to follow the solid lines in FIG. 4. On the other hand, the liquefied gas LG fed into the inner tank C is made to flow out of the tank through a liquid flow pipe E.sub.2 pushed down and dipped into the liquefied gas LG by the pressure of the gas phase G' in the inner tank C then into a gas supply line L.sub.2 of the outer piping system F. It is also known that the liquefied gas LG fed into the outer piping system F by way of a liquid feed valve b is gasified by a gas feed evaporator dd and the produced gas G is then further fed to a distribution system by following arrow G having a broken line. An air-warming type evaporator provided with aluminum fins and using air as heat source is typically used for the gas feed evaporator d.
In order to constantly supply gas G to the distribution system, the outer piping system F is provided with a pressurized pipe line L.sub.3 of a known type so that the gas pressure in the inner tank C is maintained to a constant level and liquefied gas LG is constantly fed to the liquid flow pipe E.sub.2, if the liquefied gas LG in the inner tank C is consumed to reduce the pressure of the gas phase G' in the inner tank C by the operation of feeding gas G through the gas supply line L.sub.2.
More specifically, a pressurizing automatic valve e arranged on the pressurized pipe line L.sub.3 automatically opens as the pressure of the gas phase G' drops so that liquefied gas LG flowing into the inner tank C through a pressure valve f, the lower inner tank pipe E.sub.1 and the bottom of the inner tank C is gasified by a pressurizing evaporator g and the produced pressurized gas by way of a pressure gas valve h of the pressurized pipe line L.sub.3 and a pressurizing pipe E.sub.3 of the inner tank C.
It may be needless to say that the pressurizing automatic valve e is closed whenever the pressure of the gas phase G regains a predetermined level to keep the pressure to that level. An air-warming type evaporator is typically used for the pressurizing evaporator g. Note that reference symbol i in FIG. 4 denotes a gas outlet valve of the gas supply line L.sub.2.
Additionally, a known CE as described above is provided not only with a pressurizing evaporator e for maintaining the internal pressure of the inner tank C to a constant level but also with an economizer valve k and an inner tank safety valve j for dropping the internal pressure of the inner tank C to a predetermined level. Since these components have to be actuated frequently while the cryogenic liquefied gas container is in operation, they claim up to a half of the maintenance efforts to be made for the CE and hence are believed to be subject to a number of improvements.
More specifically, the economizer valve k is connected in series with an economizer check valve m on an economizer line L.sub.4 as shown in FIG. 4 and, if the liquefied gas LG in the inner tank C is partly gasified to raise the internal pressure of the inner tank C by heat entering from outside, the gas of the gas phase G' partly flows out from the pressurizing pipe E.sub.3 of the inner piping system E along arrow EG.sub.1 and then into the inner piping system E by way of the pressure gas valve h, the economizer valve k which is automatically opened and the economizer check valve m, following the route as indicated by arrow EG.sub.2 so that it is fed to the gas supply line L.sub.2 connected to the distribution system along with liquefied gas LG to consequently drop the internal gas pressure of the gas phase G'.
Additionally, the inner tank safety valve j cannot be used to raise the internal pressure of the inner tank C if no gas is being supplied from the gas supply line L.sub.2 to the distribution system or if the economizer valve k is actuated. Thus, it is opened only when the internal pressure of the inner tank C exceeds a predetermined safety level to discharge part of the gas of the gas phase G' in the inner tank C to the outside in order to reduced the internal pressure to a predetermined pressure level.
As will be understood from the above description, when the liquefied gas in the CE is partly discharged for consumption, the internal pressure of the inner tank C falls to lower the temperature of the liquefied gas in the tank so that the pressurizing evaporator g of the pressurized pipe line L.sub.3 is actuated to recover the internal pressure to a predetermined level. If no liquefied gas LG is being consumed or it is being consumed at a low consumption rate, the liquefied gas of the CE is partly gasified in the inner tank C to raise the internal pressure thereof.
Thus, the economizer valve k and the inner tank safety valve j are actuated in order for the pressure to recover a predetermined level so that, consequently, at least one of the economizer valve k, the inner tank safety valve j and the pressurizing evaporator e has to be actuated frequently depending on the consumption of liquefied gas of the CD. Note that the pressurizing automatic valve e is typically designed to start operating when the internal pressure of the inner tank C is lower than 5.5 to 6.5 KG/m.sup.2, whereas the economizer valve k and the inner tank safety valve j are designed to start operation when the internal pressure exceeds respectively 7.0 to 8.0 KG/m.sup.2 and 9.5 to 10.5 KG/m.sup.2.
In view of the above problems of known CEs, it is therefore a first object of the invention to provide a CE comprising an outer shell and two or more than two inner tanks arranged vertically with an insulation layer disposed between any two adjacent tanks so that the CE may be installed on a limited area. If a CE comprises vertically arranged two inner tanks, the lower and upper tanks may be used to store oxygen and nitrogen respectively.
It is a second object of the present invention to provide a CE having a configuration as described above and further comprising a thermally conductive support structure connecting the lower and upper inner tanks so that the lower and upper inner tanks may contain a low boiling point liquefied gas and a relatively high boiling point liquefied gas respectively to make the internal pressure of the gas phase in the lower inner tank high and that of the gas phase in the upper inner tank relatively low in order to reduce the liquid temperature difference between the upper and lower inner tanks. As the liquid temperature of the upper inner tank and that of the lower inner tank come close to each other, the internal pressure of the lower inner tank is raised and, therefore, the lower inner tank has to be designed to withstand the internal pressure so that it can support the upper inner tank with a wide margin. Additionally, since the lower inner tank has a large wall thickness, the CE shows the center of gravity at a low position as a whole, which is advantageous from an aseismatic point of view.
Since the upper and lower inner tanks of a CE having a configuration as described above are connected by a thermally conductive support structure, it shows an improved structural strength. When the liquefied gas in the upper inner tank is consumed at an enhanced rate relative to its counterpart in the lower inner tank, the temperature of the liquefied gas in the upper inner tank will also falls to reduce the internal pressure. However, the temperature fall of the inside of the upper inner tank containing a high boiling point liquefied gas is compensated, at least partly, by the relatively high temperature of the lower inner tank containing a low boiling point liquefied gas to make the temperatures of the two inner tanks come close to each other so that the internal pressure of the gas phase of the upper inner tank is prevented from remarkably fluctuating and the rate of operation of the pressurizing evaporator of the outer piping system provided for the upper inner tank may be reduced to improve its durability and make it require less servicing.
It is a third object of the present invention to provide a CE having a configuration as described above and further comprising an inner piping system for a second liquefied gas contained in the lower inner tank and a heat exchanger section arranged in the inside of the upper inner tank so that the low boiling point liquefied gas contained in the lower inner tank may be made to pass through the heat exchanger section. With this arrangement, the temperature of the liquefied gas in the upper inner tank is raised to come close to that of the low boiling point liquefied gas contained in the lower inner tank if it falls undesirably so that it may be held to a constant level and the internal pressure of the gas phase of the container may also be held to a constant level. Consequently, the outer piping system of the upper inner tank may do well without a pressurizing evaporator that has hitherto been regarded as indispensable or, if it is used, its rate of operation may be remarkably reduced.
Finally, it is a fourth object of the present invention to provide a CE having a configuration as described above by referring to the first object of the invention and further comprising an inner piping system for a second liquefied gas contained in the lower inner tank and a heat exchanger section arranged in the inside of the upper inner tank so that the low boiling point liquefied gas contained in the lower inner tank may be made to pass through the heat exchanger section in the upper inner tank and the liquefied gas contained in the upper inner tank may also be made to pass through the other heat exchanger in the lower inner tank through another inner piping system to lower the temperature of the liquefied gas in the lower inner tank and bring it closer to that of its counterpart in the upper inner tank. With this arrangement, the temperature of the liquefied gas in the upper inner tank and that of its counterpart in the lower inner tank can be equalized to constantly maintain the internal pressures of the two tanks to a desired level.
Thus, the economizer valve, the safety valve and the pressurizing automatic valve of the pressurizing evaporator arranged on the CE will be driven less to operate so that these valves may enjoy a longer service life and the safety valve will discharge less gas so that the gases in the CE may be utilized more efficiently and effectively.